The long range objective of this project is to examine the functional importance of chromosome organization for gene expression. The chromosomes of most higher organisms are divided into two cytologically distinct domains known as euchromatin and heterochromatin. While much is known about euchromatin we know relatively little about the structure and the function of the genes residing in heterochromatin. Drosophila melanogaster is the organism of choice for these studies since sophisticated genetic and molecular approaches exist to identify and isolate heterochromatic genes. Several genes located within the heterochromatin of chromosome 2 (2Lh) of D. melanogaster will be studied. The 2Lh genes were chosen, since previous work showed that rearrangements that place these genes in distal euchromatin lead to mosaic or variegated expression. The heterochromatic light (lt) gene will be studied to: 1) determine the molecular structure of a heterochromatic gene and surrounding regions, 2) analyze the structure of its transcripts, and 3; determine the requirements for expression by analyzing mutations and introducing sequences into different chromosomal locations by transformation. Molecular probes for 1t will be used to characterize rearrangements that variegate for this gene. Mutations in genes necessary for the expression of heterochromatic genes will also be isolated. The studies of lt and neighboring genes will seek to determine whether the 2Lh genes share structural or regulatory properties relevant to their location in heterochromatin.